Direct digital printing is a significant growth area and are dealt with here.
This non-contact process allows ink to be transferred directly onto the product with no contact being made to the pack surface.
Spray coating techniques are also classed as methods of direct decoration and will be below.
The common conventional direct printing technologies used for the decoration of rigid packaging are summarized in Figure 2.1.
Figure 2.1 Summary of the common conventional direct printing technologies used for pack decoration
SCREEN PRINTING
Screen printing has been used for point-of-sale, label and industrial printing for many years.
In the industrial field, screen printing is used for clock faces, instrument panels, keyboards, signs, etc.
Screen printing is still a widely used technique for the primary decoration of both glass and rigid plastic containers, although there has been a significant decline in its use at the expense of self-adhesive labels.
Screen printing offers excellent durability, product resistance and strong bold graphics (see Figure 2.2).
Figure 2.2 Examples of direct screen printed containers
Screen printing can be a little messy and often involves flame or corona treatment of the container to facilitate ink and this is why it tends to takes place at the packaging manufacturers, such as the plastic container blow moulders or glass container makers.
There are some brand owners however who have direct printing capabilities in-house but they are few and far between.
Most brand owners and contract fillers will use containers that are pre-decorated. This eliminates the relatively slow decoration process from their operations, thereby allowing them to run their filling lines at faster speeds.
However, it eliminates the flexibility to make copy changes, and is expensive if the specific quantity ordered is incorrect.
THE PROCESS
Historically the direct decoration of containers was carried out on semi-automatic single color screen printing equipment.
Typically, the bottle or container is held by the base and the neck and is moved up to the container being printed. Screen printers are equipped with inflation systems to force air into the bottles, giving them the rigidity necessary for the screen printing process.
Multi-color printing requires registration of the image.
Custom tooling is therefore required for each bottle size and shape to engage the registration feature molded in at the bottom of the bottle. A squeegee blade moves across the screen forcing ink through the open mesh area (the image area) onto the surface creating the printed image.
A rack and pinion drive system rotates the bottle at the same speed as the screen moves during the print stroke, ensuring precision printing and multi-color registration.
The bottle is then moved to the next position where a second color can be applied. The equipment has the ability of applying multiple colors at one time, but for more complex designs the bottle can be put through the process more than once.
Normally, print cannot be on the curve of a bottle shoulder and must usually be around 7mm or so below the curve. Also, print cannot normally be placed on the bottom 20mm of the bottle.
Multi-color direct screen printing was made possible through the development of thermo-plastic (hot-melt) colors. The glass industry in particular was responsible for setting many of the standards for machine construction for container decoration.
Handling systems were generally robust to withstand the weight and stress placed on mechanical components by the relatively heavy glass bottles. The same capabilities were soon extended to plastic bottle decoration.
Over time there has been a trend towards the use of fully automatic bottle decoration with multi-color capability. Although solvent based inks are used, the introduction of UV curable ink systems heralded a new era for the direct decoration of bottles.
In the glass industry inks that are applied directly to the surface of a bottle are fired through a Lehr furnace. After firing at temperatures of up to 1180° F the label design is permanently fused to the glass.
Today’s automatic screen printing equipment offers multi-function capabilities such as pre-treatment for polyethylene and polypropylene bottles by flame or corona discharge which facilitates ink key. The equipment may also need to interface with other operations such as bottle molding or filling.
Equipment must be versatile enough to handle a variety of container shapes and sizes, the registration of requirements of high speed multi-color printing and a wide variety of substrates and inks.
The plastic bottle printer must often inflate his substrate/container to make it rigid enough for printing.
CHARACTERISTICS OF DIRECT SCREEN PRINTING
The characteristics and benefits of direct screen decoration differ and are dependent on the market and packaging material.
Under-the-sink household products for example tend to use medium density PE bottles for products such as washing up liquids, detergents and bleaches. Featuring simple designs of one or two colors, containers tend to be printed using solvent based inks with infrared/hot air drying.
Economy, performance on flexible materials, product resistance and durability are the benefits delivered by the screen process in this sector. Pack size will tend to be large and the printing will often be carried out by the blow-molder in this market.
In the toiletries and cosmetics market the requirement is for a much higher quality of decoration. Typically print will be 3-4 colors onto HDPE containers. In this sector UV ink systems have superseded conventional solvent based systems.
Modular in-line UV printing equipment allows instant drying between stations and can achieve a high output of printed containers per hour.
The advantages of UV screen printing in the personal care market are as follows;
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Good graphic definition and detail on text
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Excellent product, durability and performance in wet and humid conditions
It is worth noting that the trend towards PET/PETG containers is offering opportunities to other decorative systems such as self-adhesive labels and sleeving.
In the glass industry high temperature long stove screen inks are used particularly for the decoration of high quality toiletries, cosmetics and spirits brands. Four color process printing on glass is possible when using UV inks whilst other effects can be achieved using precious metals, glow-in the dark inks, and raised lettering.
Screen printing is also used for decorating returnable (or premium brand one-way) glass bottles and more recently PET bottles – including Coca Cola and Pepsi bottles.
For returnable bottles the bottle is screen printed at the point of manufacture with a durable, scuff resistant, image that will endure numerous round trips, including washing operations prior to re-filling. Screen printing of glass or plastic bottles is relatively slow at around 6,000 bottles per hour and could not be carried out on a bottling line. However, if used for decorating returnable bottles, where there may be up to 20 trips per bottle, the process becomes a viable operation and can compete with labeling.
By using different thickness gauge meshes, mesh spaces and inks it is possible to print ink films thicker than any other process, so creating a tactile feel and image to the decoration. With some stencils it is possible to achieve an embossed feel to the decoration.
Four color process printing on glass is possible when using UV inks whilst other effects can be achieved using precious metals, glow-in the dark inks, and raised lettering.
DRY-OFFSET PRINTING
Dry-offset printing (known as offset letterpress) was developed in the early 1960’s with the introduction of photopolymer dry offset plates.
The term dry-offset derives from the fact that these new plates could be used on standard litho presses without the use of the water that is needed for conventional litho printing.
With dry-offset the complications of maintaining the ink-water balance associated with conventional lithography are eliminated. With dry-offset however it is not possible to print as small a dot as it is with lithography, so print quality and definition can suffer.
THIN WALL PLASTIC CONTAINERS
Dry-offset letterpress printing is used predominantly for the decoration of thin walled plastic containers in the dairy sectors (fats, ice cream, yogurt, buckets etc).
Offset letterpress uses a photopolymer relief plate (similar to conventional letterpress printing). Unlike conventional letterpress where the ink is transferred directly to the substrate from the plate, in dry offset inks are applied indirectly via a rubber blanket and then onto containers that are held on a rotary mandrel system. On a mandrel system the containers are supported internally to provide a stable surface on which to apply the print.
Due to print quality issues associated with dry-offset there is a trend to in-mold labeling in this sector.
TWO PIECE CANS
The aluminium two-piece can used for soft drinks and beers is printed dry-offset by the container manufacturer.
As with thin walled plastic container decoration, a special plate prints directly onto the blanket of an offset press and the blanket then offsets the image onto the metal.
The inks are only allowed to dry after this transfer.
Dry offset printing for conical/cylindrical packaging, such as beverage and aerosol can decoration, places multiple inks, 'wet-on-wet', on a common blanket and that image is then transferred to the packaging to be decorated. The inks are only allowed to dry after this transfer.
The introduction of High Definition (HD) letterpress with UV exposure and high resolution imaging is overcoming some of the challenges for dry offset printing ie. smooth highlights, clean printing of text; and fine positive and negative linework (Figure 2.3).
Figure 2.3 Dry offset printing on cans using Esko HD letterpress
PAD PRINTING
Pad printing (also called 'tampography') was developed principally as an economic method of printing awkward, irregular or 'not flat' profiles. It is used for printing on 'difficult to print' products in many industries including medical, automotive, promotional, packaging, apparel and electronic objects, as well as appliances, sports equipment and toys.
Pad printing can also be used to deposit functional materials such as conductive inks, adhesives, dyes and lubricants.
The heart of the process is the printing pad which is made of silicon rubber. It is malleable and therefore able to mould itself to the contours of the area to be printed.
THE PROCESS
Pad printing is a process whereby an image is transferred from an etched plate via a silicone pad, directly onto a component surface.
Physical changes within the ink film allow it to leave the etched image area in favour of adhering to the pad and then it is released from the pad and transferred to the substrate.
The pad printing process is described in Figure 2.4.
Figure 2.4 Typical pad printing process
A-B - The inking slide moves forward, the self setting spatula scoops ink from the reservoir and deposits an even layer across the plate.
The doctor blade scrapes the ink back from the plate leaving a deposit of ink in the etched plate surface. Solvent evaporates from the ink and its surface becomes tacky.
C - The pad descends and comes into contact with the ink in the image area where it transfers.
D - As the pad travels to the target area solvent evaporates allowing the ink on the pad to become tacky.
E The pad descends to the target area which has greater adhesion with the ink than the silicon rubber pad. As the pad lifts from the target area the ink adheres to the substrate.
SPRAY COATING
New developments in the electrostatic spray coating of glass and most recently, PET bottles now look to provide some interesting new possibilities for added-value bottle decoration.
It is also suitable for many other products including cosmetic containers, tableware, lighting glass and decorative glass.
With electrostatic coating technology bottles are automatically sprayed with an electrostatic liquid or powder, giving a range of effects from transparent to opaque, full gloss to matt, silky smooth to stone effects, slippery sleek to rubbery soft, as well as thermochromic or fluorescent images.
Such effects can also provide for a glass etched or frosted look, offering bottles that are colored or textured and which are designed to sell and add value to a product.
The various coating options can all be over-labeled or screen printed.
Coatings are extremely thin and can be recycled with the bottle. Being water-based, the coatings are compatible with organic inks and reduce the amount of overall packaging used.
By combining spray coated barrier coatings with special effect spray finishes and then labeling (which can make frosted or etched effect areas transparent if required), then electrostatic spray coating could provide bottlers with some very exciting added value effects for, say, speciality niche beer brands.
TRANSFER DECORATION
Transfer decoration is a method of product decoration which is an alternative to printing directly onto the container or using conventional labeling systems.
Heat transfer decoration uses a combination of heat and pressure to carry an image onto a product via a liner. The result is the look and feel of a label without a label.
Heat is typically required to soften the inks, adhesives and substrate surface to ensure the correct adhesion.
There are a number of heat transfer systems available, the most common of which will be dealt with in this section (Figure 2.5). Each transfer decoration method requires a different type of machine to successfully apply a crisp, quality image to the product or pack.
Figure 2.5 Main transfer decoration technologies
SCREEN PRINTED TRANSFERS
First introduced in the early 1960’s screen printed heat transfers evolved as a decorative method in the plastic molding industry for adding color graphics to consumer electronics and appliances.
Screen printed heat transfers deliver outstanding graphic quality with excellent ink coverage and utilizing custom chemistry designed for the specific material and application.
Screen printed transfers produced in layers (one for each color), that begins with the top color. In some cases a layer of adhesive may be required to adhere to specific substrates.
Printing is typically onto a polyester carrier.
The inks used in heat transfer applications are manufactured to a specific formulation that delivers the performance characteristics required for each application.
An applicator uses a combination of heat, pressure and time (dwell) to transfer an image onto a product via a liner.
WAX RELEASE HEAT TRANSFER LABELING (THERIMAGE®)
The Therimage® (Wax Release) process was first introduced by Dennison Manufacturing Company around 1960.
Therimage or Heat Transfer Labeling (HTL) is a method of product decoration which is an alternative to printing directly onto the container or using conventional labeling systems.
The reverse printed graphics are printed onto a special release coated paper. The label construction consists of adhesive lacquer, ink layer, protective lacquer and the release liner.
During the automatic application process only the printed inks or film lacquer transfer to the product.
Therimage will give the same look as the other systems but without using a conventional label substrate and when applied to the container looks as if it has been printed directly on to the container surface and not actually applied as a label.
THE PRINCIPLES OF THE PROCESS
There are two separate elements involved in the printing and application of Therimage/HTL decoration.
Firstly, the printing and laquering process and secondly the transfer and application of the printed layer (label) onto the container.
Therimage labels can be printed using the mainstream conventional printing processes, gravure, flexographic, offset litho and screen. The inks are compatible with all these processes and produce excellent graphics, with the screen process perhaps giving the best result when good opacity is required.
The image is printed in reverse which means that the colors which are normally printed CMYK. i.e. black as the last color would be printed KYMC.
i.e. cyan as the last color printed, this means that the 'label' lays upside down on the substrate carrier. After the printing has been completed a lacquer coating is applied which provides the adhesive layer which secures the Therimage label to the container.
The carrier substrate is also treated with a special wax release coating which allows easy release of the label from the carrier to the container.
Figure 2.6 shows the structure of a heat transfer label.
Figure 2.6 Structure of heat transfer label
The second stage is the application of the label onto the container, this involves passing the printed web through an applicator machine which transfer the label from the carrier web on to the container (see Figure 2.7).
Figure 2.7 Therimage decoration application process
By the use of pressure and controlled heat, the printed ink layers of the Therimage label are placed in position onto the container giving a permanent and robust decoration.
A pre-heat station is required to allow the inks to soften so that they 'flow' from the carrier to the product.
It is sometimes necessary to flame treat the containers prior to the label being applied, this process oxidizes the surface of the container to increase the surface energy, not unlike corona treating a conventional substrate prior to printing.
Recent developments in Therimage ink systems have created inks which do not require the flame process to be used, this ‘flameless’ technology allows the adhesive to adhere to the container without the use of the flame process.
Advantages
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Therimage labels give excellent graphic reproduction include fine vignettes
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Multi-color printing including metallic and acid-etch finishes.
Disadvantages
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Some containers require a flame treatment.
DIGITAL HEAT TRANSFER DECORATION
Patented systems are currently evolving that use digitally printed transfer labels to decorate non-flat or shaped products made from plastics and metals, glass and ceramics (Figure 2.8).
Figure 2.8 CDigital uses Xeikon digital print technology to apply imagery to non flat products
Digital transfers are used primarily for the decoration of consumer goods packed in plastic containers such as seamless tubes, buckets, cartridges, etc.
The system uses dry toner to print digitally onto the underside of patented constructions. The special heat transfer carriers are constructed with a release layer and a lacquer layer. The converter lays down a water-based adhesive on top of the toner in-line with the press.
The roll of printed transfers is then fed into an applicator which applies the label onto the container using heat and/or pressure in a process similar to hot foil blocking.
With digital transfers, heat is required to activate the adhesive coating of the transfer along with the toner printed image.
Digital heat transfers offer excellent, reliable control and the ability to incorporate variable data into a long run.
Containers made from a range of plastics including polypropylene (PP) and high density polyethylene (HDPE) can now be decorated with high impact graphics using this method (Figure 2.9).
Figure 2.9 Digital heat transfer onto plastic containers. Digital decorating system jointly developed by Xeikon and Italy-based Moss
SUMMARY
Heat transfer decorating offers a number of benefits over many other decorating processes, especially wet decorating processes.
Some of the key advantages are summarized below;
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Dry Process with no wet inks, chemicals etc.
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No discernable label edge
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Multiple colors in a single pass application
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High scratch and chemical resistance
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Permanent with fade resistance
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Custom colors and formulations specific for each application and substrate
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Decorated plastic containers can be squeezed or deformed without adverse impact on image quality or adhesion.
HOT FOIL BLOCKING
Hot foil blocking is a common embellishment process used in label manufacturing. A similar process can be used for creating decorative metallic effects directly onto to the surface of a product, bottle or jar.
Hot foil blocking is a process that uses metallic dies with a design or lettering etched on to the surface. During the foiling process the colored metallic pigment coating from a ribbon of plastic material known as the ‘carrier’ is transferred onto the product surface.
The hot foil machine applies pressure and heat to the metallic block, which activates the film. This causes a thin layer of foil to adhere to the surface of the product.
